1. Field of the Invention
This invention relates to a color video signal mixing system which mixes two color video signals to obtain a new mixed color video signal.
2. Description of the Prior Art
The conventional methods for mixing two color video signals include a simplified method which is carried out by directly adding color video signals in the form of television signals.
FIG. 1 of the accompanying drawings show a rotary head type video tape recorder (hereinafter called VTR) which is arranged to obtain a mixed color video signal in accordance with the above-stated simplified conventional mixing method and to record the mixed color video signal. The illustration of FIG. 1 includes a magnetic tape 1; a cylindrical tape guide drum which is arranged to have the tape 1 obliquely wound approximately 189.degree. around it; a supply reel 3 for feeding the magnetic tape 1; and a take-up reel 4. The magnetic tape 1 is arranged to travel in the direction of an arrow 5 and the drum 2 in the direction of an arrow 6. The VTR includes heads 7 and 7' which are arranged for ordinary recording and reproducing operations and heads 8 and 8' which are arranged for recording a mixed color video signal. In ordinary recording, a color video signal received via an input terminal 10 is supplied via a selector switch 11 to a recording signal processing circuit 12. The circuit 12 processes the color video signal into a signal form suited for magnetic recording. The processed video signal is then supplied to the heads 7 and 7' via another selector switch 13. The heads 7 and 7' then record the video signal on the magnetic tape 1. In this instance, the switches 11 and 13 are respectively connected to their terminals R. In ordinary reproduction, switches 13 and 15 are respectively connected to their terminals P. A reproduced color signal obtained by the heads 7 and 7' is supplied via the switch 13 to a reproduced signal processing circuit 14. The circuit 14 then returns the reproduced color video signal back to the original signal form. Then, the thus processed reproduced signal is produced via an output terminal 16.
FIG. 2 shows the condition in which the signal is recorded on the magnetic tape 1 by the VTR shown in FIG. 1. One field portion of the color video signal is recorded within each of recording tracks 19A, 19B, 19C and 19D. Assuming that the signal is recorded in the track 19A by the head 7, the signal is recorded in the track 19B by the head 7' and then again in the track 19C by the head 7. Further, while the head 7 traces the track 19C in the position as shown in FIG. 2, the head 8 is in a position to trace the same part of the adjoining track 19D. The heads 7' and 8' are also arranged likewise relative to each other.
For recording a mixed color video signal with these heads 8 and 8', the VTR operates as follows: The switch 13 is shifted to the terminal P while switches 11 and 15 are respectively shifted to the terminals M. A color video signal which is reproduced by the heads 7 and 7' is supplied via the reproduced signal processing circuit 14 and the switch 15 to an adder 17 and is added to another video signal which is supplied via the terminal 10. In this instance, the servo rotation of the drum 2 is performed according to a vertical synchronizing signal separated from the video signal supplied via the terminal 10. By virtue of this arrangement, the reproduced video signal and a video signal coming via a terminal 16 coincide with each other. A mixed color video signal obtained at the adder 17 is changed by a recording signal processing circuit 18 into a video signal form suited for magnetic recording. After that, the mixed signal is recorded by the heads 8 and 8' on the tracks which adjoin the tracks being traced by the heads 7 and 7' and which have already been traced by the heads 7 and 7'. However, mixing color video signals in accordance with this method tends to create a moire of color or a change in hue. This problem of the prior art method results from difficulty in bringing the phase of one chroma signal into perfect agreement with that of the other. This is also because two added chroma signals being about a phase deviation or a hue deviation.
To solve this problem, it has been the practice, in TV broadcasting or the like, to mix two color video signals by switching them from one to the other according to the level of the luminance signal or the phase (hue) of one of the two color video signals.
However, in forming a signal for effecting the switchover according to the luminance signal level, for example, a part of one of the color video signals which is to be left unmixed must be provided with a background of high luminance (white) or low luminance (black). This purpose may be attained during an image sensing operation by applying a spotlight on a desired object or by sensing the image with a white wall used as a background. However, such an arrangement is quite troublesome. Besides, since the video signal must be subjected to a switching operation, some part of the video signal that is discarded by switching eventually disappears in the mixed video signal thus obtained. Then it is hardly possible to attain an overlapped picture effect.
Further, in the event that one of the color video signals that is to be used for the preparation of the switching signal happens to be a signal close to the threshold value for switching control, each picture would include too many switching parts and thus its quality would be excessively degraded.